A preliminary view of Indian affairs will enable us to understand this commencement of the war. By the combined counsels and schemes of the British agents, and some of the principal chiefs among the Indians, the seeds of hostility were sown among them soon after the peace of Greenville, and were gradually nurtured into war.
At that time, Little Turtle and Blue Jacket were the leading chiefs among the northwestern tribes. They had disagreed about the manner of opposing [General Anthony] Wayne s army. The plan of Blue Jacket was adopted, and eventuated in the total defeat of the Indians, as predicted by the other. After this event, Little Turtle continued friendly to the United States. He was of opinion, that the Indian tribes were unable to contend against the Americans; that no material aid would be furnished them by the British; and that would only be the means of their losing more of their lands. Blue Jacket had more confidence in the British; he thirsted for revenge against the Americans; and he wished to regain the lands which had been ceded by the treaty of Greenville. [Source]
Also see the Aug 2013 post, Native American And British Influence At The Close Of The Revolutionary War.